The plot device of Oliver handing the mantle of Green Arrow to John Diggle helped free up Stephen Amell's schedule so he could film his guest appearances for the Crisis on Earth X crossover.
As Felicity is working on a name for her start-up company, the name "Oracle" can be seen crossed out on her transparent marker board. This is not only a reference to the computer company of the same name, but the DC superhero (Barbara Gordon, also known as Batgirl) who is an occasional inspiration for Felicity's storyline.
Kord Industries, seen in this episode, was a company owned by Ted Kord aka The Blue Beetle in DC Comics.
In the DC comics Onyx is a former member of the League of Assassins who became a vigilante after swearing to never kill again. She has appeared besides Green Arrow and Batman.
This is the first time we see someone other than Oliver wearing the season 5 Green Arrow suit.